| Modern urban buildings are expected to be advanced and reliable in the following aspects: firstly,the structural performance should be satisfactory during major disasters/emergencies including intense earthquakes,in order to avoid heavy and comprehensive loss;secondly,modern cities require buildings to be architecturally appealing.Suspended buildings are reasonable options as far as those aspects are concerned.Tensioned vertical components with smaller sections lead to visual transparency;large open space on the first floor makes the building suitable for high-end offices and headquarters.The suspended parts,which are usually termed as secondary structures,serve as large tuned-mass dampers(TMDs).This passive control scheme becomes more efficient in attenuating primary structure(PS)displacement and secondary structure(SS)accelerations,as more mass is transferred from the primary to the secondary structures,within a certain range of mass ratio.However,one major concern is that suspended building structures have yet to fulfill its full potential of vibration control because of the fragility originating from the excessive inter-story drift of its SS during earthquake.The author proposes an innovative subtype of suspended building structure system to overcome this fragility and further improve its lateral aseismic performance.This subtype takes advantage of modular construction techniques.The SS consists of suspended discrete modules that provide protection to nonstructural members against excessive inter-story drift.Additionally,springs that provide inter-story stiffness remain elastic.These effects extend the inter-story drift limit of SS and make sufficient relative motion possible.Thus the tuning-dissipation mechanism of the primary-secondary structural system can be further optimized.Major aspects of the carried out research are listed as follows.1)The frequency-domain response characteristics are analyzed and optimized.Firstly,a continuous numerical model is established by the Euler-Bernoulli beam assembly and the corresponding equilibrium differential equations are solved numerically.The frequency-domain dynamic response curves subjected to ground acceleration and wind load excitation are derived in parametric forms.The key parameters(including,stiffness ratio,mass ratio,damping distribution/type,damping value,and the number of secondary structure segments)are analyzed to summarize the tuning mechanism between the PS and SS.The potential of multi-mode tuning is revealed.Secondary,an eleven-story two-dimensional numerical model is established.Three sub-configurations of different attenuation strategies are set.The mean square values of the frequency-domain response are set as the objective function.The single-/multi-objective optimizations of the inter-story stiffness and damper damping coefficient of the SS are conducted.The result shows that the displacement response of the PS reaches 0.45 of that of the uncontrolled structure,but the inter-story displacement demand of the SS could reach 15 times that of the PS,showing the necessity of modularization.Frequency-domain transfer functions and complex modal information reveal the superiority of the sub-configuration with inter-module dampers.As a modular secondary structure allows vertical irregular distribution of structural parameters and responses,six levels of irregularity constraints are set for the distribution vector of the stiffness and damping parameters.The single-/multi-objective optimization is performed.It is shown that as the irregularity constraints are relaxed,the bending moment of the primary structure can be further reduced by 50%.Additionally,non-stationary dynamic analysis and time-frequency domain optimal result comparison compose a thorough verification of the first-stage conclusion of the research.2)The aforementioned control effects and mechanisms are verified by shake-table tests.The specimen is scaled 1:15;the T-shaped steel structure is used as the PS;the swaying mechanism is formed by the slotted steel plates + round steel pins + connection parts on suspended modules;Adjustable airpot-dampers and mechanical springs are used to form the inter-story connections.For three sub-structure configurations,18 models with different parameter combinations and 2 non-control models are set according to the pre-optimized results.It is proved that the system has reduced the vibration by more than 50% and has a quick decay of the vibration.The representative model among each configuration is selected.The optimal model reached an inter-structure displacement of 3.75% when subjected to 0.1 g PGA excitation.A modeling strategy based on experimental phenomena is proposed,essentially by setting regional Rayleigh damping and deducing its value by an observation-assumption-verification process.The numerical model of the structure and the dampers are calibrated separately,before the two parts are combined to obtain the complete structural model.A satisfactory match without further adjustment is automatically attained in terms of time-history response;this has validated the modeling strategy.3)The impact of the post-yield behavior of the primary structure is examined.Based on the experimentally validated modeling strategy,a series of numerical models with different story numbers,damper types,and nonlinearities of the main structure are established.Tri-objective-three-parameter/dual-objective-two-parameter nonlinear time-history optimizations are performed.Each of the aforementioned factors shows a notable impact.It is revealed that the tuning between the primary and secondary structures weakens when the primary structure enters the elastoplastic stage.Meanwhile,the optimal values for the stiffness of secondary structure and damping coefficient decrease.4)The fragility analysis is conducted to quantitatively evaluate the overall performance.Uncertainties in both excitation and structural parameters are considered.The far-field motion recommended by FEMA P695 was adopted in multi-strip method(MSA)analysis.Multiple limit state functions are defined and a specialized component-group-system-level mapping rule of limit states is proposed to account for distribution of vulnerability.It is proved the advantages of the innovative system compared to ordinary suspension structure,frame structure and frame structure with viscous damper(in 50-year return period,the probability of exceeding the repairable limit state is reduced to 23% of the ordinary suspension structure / 40% of the frame structure with viscous dampers,and that of the collapse limit state is reduced to 50% of the ordinary suspension structure / 71% of the frame structure with viscous dampers).Reliability-based optimizations with 3D numerical models are carried out to minimize the weighted average of the exceedance probability of the four limit states in the 50-year design cycle.The optimal parameters,the Pareto front distributions,the optimized fragility curves,and the nonlinear time history features,etc.,are thoroughly analyzed for three groups of eight influencing factors: 1)excitation,2)energy dissipation,and 3)practical issues(including hysteresis behavior inside the module,the collision and the fuse-type connection between the primary and secondary structures).It is proved that the conclusions of 2D domain can be generalized to 3D domain;the collision between primary and secondary structure and fuse-type connection poses a notable handicap on the structural performance.Collision is the primary handicap(pounding gaps of 1 m width(at least)are recommended),and the intriguing displacement of the fuse connection is more impacting than the stiffness of the fuse connection. |
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